The extension of life expectancy in modern societies has been accompanied by a new concept which is also a challenge: that of “optimal aging”. We now know how to live longer, but do we know how to live healthy longer? Age is, in fact, the main risk factor for the development of neurodegenerative diseases, or dementias, of which Alzheimer’s disease is the most common. Dementias are accompanied by cognitive decline which gradually leads to a loss of autonomy; this represents a considerable emotional and financial burden on those affected, their families and society. As current pharmacological treatments are inconclusive, forecasts for the future are particularly alarming: globally, 75 million cases of dementia are predicted in 2030, and 132 million in 2050; this equates to a new case every three seconds, and will cost US $ 2 trillion in 2030. To prevent cognitive decline it is important.
Yet not all older people have dementia. In fact, there are considerable inter-individual differences in how aging affects the brain and cognitive functions. Also, among people who develop dementia, not only does the age at onset of brain damage vary greatly, but research has also found that with the same degree of severity of lesions, some people stay much longer behaviorally functional than others. Thus, some dementias can even remain “silent” or asymptomatic: their diagnosis is made only after death, on the basis of biomarkers (amyloid deposits or tau protein), without the affected person having been significantly affected in his everyday operation.
The Concepts of Brain Reserve and Cognitive Reserve
This form of resistance or resilience was conceptualized under the general term reserve in the early 2000s and is today considered to act on two levels:
on the structural level, by guaranteeing a good “neurobiological capital” (the number of neurons, synapses, and other structural characteristics of a healthy brain), and this, by offering a larger capital at the beginning, or by contributing to a better preservation of this capital that is to say that we lose little of it, or, in any case, less than the average. It is then a question of cerebral preservation, or brain maintenance.
In terms functional, allowing a form of adaptability or flexibility for optimal use of neural networks available, which will be able to spontaneously adjust to continue to perform cognitive tasks even despite a reduction or impairment in structural capital called cognitive reserve, or cognitive reserve.
What Control Do We Have Over This Reserve?
Among the factors which thus modulate the brain damage associated with aging, or the relationship between this damage and cognitive functioning, some are innate and cannot be modified. This is the case for certain structural aspects (such as brain volume), or genetic background, such as the apolipoprotein E ε4 (APOE4) genotype, which would increase the chances of developing Alzheimer’s disease.
However, there are many factors influencing reserve that seem to relate to our lifestyle and therefore over which we have some control. It is estimated that 21 to 35% of Alzheimer’s cases are attributable to environmental factors, meaning that about one in three cases could be avoided if we act on these factors.
Last Words
The benefits of controlling some of these factors are sometimes more pronounced in carriers of the APOE4 genotype, suggesting that an adapted lifestyle could help to compensate or neutralize a genetic risk factor. It is also important to consider that, given that age is the factor most correlated with the development of dementia, anything that delays the onset of the disease contributes to reducing its prevalence. Thus, delaying the onset of the disease for five years would correspond to reducing its overall prevalence by approximately 40% by 2050. Overall, these observations therefore bring prevention strategies to a privileged position in the fight against this scourge.
